Hmm, I tend to play stiffer polys with gut to temper the bed but three strings I'd research/look into are Pro Line II, the Head Sonic line, and for the softest end PolyStar Energy.

I'd stay away from PS Energy as a cross to gut... it loses tension too quickly and I found it to be way too unpredictable with my gut. Every day was a different experience, not what you want in a string.

I would honestly say MSV Co-Focus is an amazing cross to gut. Not only due to all the threads about it, as I've used it too, and it really is soft and predictable. It does soften over time as well, which most polys tend to do the opposite, but this means you can string it at or near the tension you like your gut, knowing the co-focus will get softer. I'm not a fan of stringing crosses more than 3lbs lower than my mains, even though its ok to do so.

If it weren't for the 16x15 pattern you are using, I'd suggest WC Mosquito Bite as the all-time great soft poly cross. It holds tension wonderfully, but that's going to break fast in the 99S. (only comes in 1.16mm)

Co-Focus comes in many gauges and colors, so you can tweak it to your needs. 1.27mm is their thickest offering, which is also the best for tension stability, and still nice and soft and spinny. They also make "Focus Hex Soft" in 1.30mm. I've not tried it yet, but it's probably nice too, as regular Hex is definitely stiffer than Co-Focus.